Lamp assemblies utilizing tungsten filament incandescent lamps are relatively inefficient. In fact, a conventional 40 to 100 watt light bulb utilizing a tungsten filament provides only about 4% to 6% visible light based on the used electric power. The remainder of this used electrical power is mostly converted to heat. This generation of heat can result in dangerously high temperatures. For example, the temperature of many typical frosted glass enclosures for conventional incandescent lamps can easily reach 200.degree. F. to 250.degree. F.
As a conventional incandescent lamp operates, the generated heat causes the tungsten atoms to evaporate from the filament, resulting in it becoming thinner. This evaporation of tungsten may also result in dark deposits of metallic tungsten on the inside of the glass enclosure. As the filament becomes thinner, it eventually breaks. The lifetime of a conventional incandescent light bulb is roughly 700 hours.
Alternatively, tungsten halogen lamps contain iodine, bromine, or chlorine gases or mixtures thereof. These lamps can operate at higher filament temperatures than conventional tungsten lamps because the evaporated tungsten atoms combine with the contained halogen gases, circulate within the lamp, and re-deposit on the filament. The lifetime of tungsten halogen lamps is thereby increased considerably relative to conventional incandescent lamps. In addition, the higher filament temperatures produce whiter, brighter output light at higher color temperatures. Tungsten halogen lamps also operate more efficiently than incandescent. Typically tungsten halogen lamps operate at about 7% to 9% efficiency. However, due to higher operating temperatures, fused quartz glass is required for the enclosure. These glass enclosures must be able to withstand temperatures of approximately 500.degree. F. to 700.degree. F. Also, there is a hazard of exposure to halogen gases if the fused quartz glass enclosure breaks. For cost reasons and to insure high operating temperatures which enable efficient circulation of tungsten-halogen within the enclosure, tungsten halogen lamp enclosures are purposely made small. For example, high wattage tungsten halogen lamps have fused quartz enclosures shaped like a pencil around a linear filament.
A third type of lamp which has been recently developed is a miniature xenon lamp. These lamps operate at lower filament temperatures and can therefore utilize conventional glass enclosures. Xenon flash tubes (for cameras) and xenon strobe lights (for timing lights on reciprocating engines, etc.) have been used for decades in applications where very high light intensity and very white light output is required. The newly developed miniature xenon lamp efficiently produces output light in the visible spectrum from noble gas xenon atoms excited by the tungsten filament inside the lamp. Activated xenon atoms produce an even whiter, brighter, higher color temperature light than tungsten halogen lamps. Xenon lamps also operate at a higher efficiency than tungsten halogen, typically at 10% to 12% efficiency. Compared with tungsten halogen, the xenon lamp also produces much less ultraviolet light component, where such ultraviolet light is highly undesirable. In fact, the spectral emission characteristics of activated xenon atoms show that only 4% of the output light is in the ultraviolet wavelengths below 0.4 microns, 71% is in the visible spectrum from 0.4 to 0.7 microns, and 25% is in the infrared wavelengths above 0.7 microns. Reduced filament temperatures in the xenon lamp, combined with the presence of the safer noble gas xenon, allows the tungsten filament in the xenon lamp to operate for significantly longer times, than even the tungsten halogen lamps.
Different types of low wattage lamps are compared in the following table:
______________________________________ Incandes- Tungsten Characteristic cent Halogen Xenon ______________________________________ Typical Wattage Rating 40-100 W 10-50 W 5-35 W Typical Filament Lifetime 700 hrs 1500 hrs 10,000 hrs Type of Filament tungsten tungsten tungsten Light-to-Electricity 4-6% 7-9% 10-12% Efficiency Potential Halogen Gas Hazard no yes no Type of Enclosure glass fused glass quartz Enclosure Surface 225.degree. F. 550.degree. F. 475.degree. F. Temperature Typical Power Supply Voltage 120 volts 12 volts 12 volts Typical Color Temperature 2400 K 2700 K 3000 K ______________________________________
Low voltage (typically 12 volt) tungsten halogen lamps are so small and so bright that small glass reflectors with dichroic reflective coatings and multiple facets have become widely used to provide a directed beam of output light. These glass reflectors are called MR-11 (in 12 volt ratings from 5 watts to 35 watts with G4 bi-pin lamps) and the MR-16 (in 12 volt ratings from 10 watts to 100 watts with G5.3 and G6.35 bi-pin lamps). The MR-11 reflector is only 35 mm in diameter and 27 mm long, with 1.0 mm diameter by 6.0 mm long connection pins spaced 4.0 mm apart. The MR-16 reflector is somewhat larger, but still only 50 mm in diameter and 38 mm long, with 1.5 mm diameter by 7.0 mm long connection pins spaced 5.3 or 6.3 mm apart. Since tungsten halogen lamps are significantly brighter than incandescent lamps, less wattage is needed for the same illumination, and the extremely small size is a great advantage in locating these types of light fixtures where incandescent lamps would not physically or aesthetically fit.
Many types of directed-beam lighting systems using tungsten halogen lamps have come into wide use. Track-mounted systems (i.e. located on ceilings) are very popular because individual lights can be directed where better illumination is desired. None of these types of track-mounted directed-beam lighting systems using tungsten halogen lamps are waterproof. None of them can be used where water might come into contact with the lighting system.
Although small in size and high in brightness, one problem with tungsten halogen lamps is the relatively high temperatures at which tungsten halogen lamps operate. For example, with a 12 volt, 10 watt tungsten halogen lamp in an MR-11 glass reflector, the temperature of the outer surface of the MR-11 reflector itself operates between 350.degree. F. and 400.degree. F., which is much too hot to touch without burning and which can also cause scorching or burning of adjacent materials.
Typically, metal housings are used to enclose these lamp-reflector assemblies. Air cooling slots may be provided to vent some of this excessive heat. Even so, fingers or hands are still easily burned when touching the housing such as when adjusting the direction of the output light beam or when changing the position of a tungsten halogen lamp fixture mounted on a goose neck. Therefore, there is a need for a lamp assembly which can provide a high intensity directed output beam without having a high temperature enclosure.
During the investigation which led to the discovery of this invention, we attempted to develop an enclosure for a lamp-reflector assembly which would be safe to touch and handle, but which would also be waterproof because of the variety of applications for small lighting systems where the waterproof feature would provide distinct advantages over other presently available lighting systems. The investigation started with tests to verify the performance of commercially-available waterproof lighting systems using tungsten halogen lamps.
Initial tests were performed using a 12 volt, 10 watt halogen lamp. This lighting system consists of a solid transparent (Pyrex) test tube, approximately 0.375 inch in diameter, with a rubber stopper through which the electrical connection wires pass to the tungsten halogen lamp itself. During these tests, a hole was burned in a living room carpet due to the very high surface temperature of the Pyrex test tube enclosure. Although the lamp met the waterproof requirement, it suffered from very high and unacceptable temperatures when operated in ambient air. Also, the output light is unshielded and there is not a directed output light beam. A directed output light beam is much more desirable.
Additional tests were also performed using a similar halogen lighting system. In this test, a 12 volt, 20 watt tungsten halogen lamp and MR-11 reflector is contained within a solid cylindrical plastic enclosure, approximately 2.2 inches in diameter by 2.2 inches long. The enclosure has a screw-on (1/4 turn) removable cover that contains a translucent polycarbonate plastic lens, which can be exchanged to change the color of the output light. When the cover is screwed on, the plastic lens is forced against a square thermoplastic gasket that is supposed to provide a water seal. However, changing the plastic lenses was difficult by hand and even with the aid of pliers was not easy. Changing the color of the output light is not convenient. Unfortunately, after operating the test lamp for some time, the durometer rating of the gasket material became higher, and there was insufficient compression to enable a good water seal. Water leaked into the housing despite attempts to smooth the injection molding imperfections in the plastic sealing surfaces. Several tungsten halogen lamps then rapidly failed due to either thermal shock of the fused quartz lamp enclosure or electrolytic corrosion damage of the 12 volt terminal pins.
These tests, along with an investigation of several other types of underwater lighting systems having similar problems, show that there is a need for an improved low-wattage lighting system for producing a directed beam. Such a lighting system would be capable of using tungsten halogen lamps, xenon lamps or other high-output lamps in small lamp-reflector assemblies to provide output light in a directed beam. Such a lighting system would preferably provide for optionally changeable colors housed in an enclosure which would minimize the risk of burn or fire. There is also a need for such a light assembly which could be operated underwater or at least in wet environments.